The invention generally relates to power conductor rails used for electrical rail transportation systems such as metro transit rail vehicles, people movers, heavy rail commuters and the like.
Electrically powered rail vehicles have long been used for mass transit systems. Electric rail systems typically employ a three-rail configuration, the rail system having two running rails to support the vehicle and a third rail to conduct the necessary electrical power. In early electric rail systems, all three rails were made of standard steel, each rail being identical in configuration. In the late 1960's, metro transit authorities and supporting manufacturing companies began experimenting with different rail structures for third rails in an effort to reduce electrical resistance and reduce weight for ease of handling and installation.
One of the first improvements was the use of an aluminum cladded rail whereby prefabricated extruded aluminum sections were bolted or clamped onto each side of a conventional steel rail web. An electrical conductor rail having non-ferrous metal extrusions secured to both sides of the steel web of the rail using bolts is disclosed in U.S. Pat. No. 3,730,310. In this structure, the aluminum cladding was preselected and secured to the steel rail web in the field by the installation personnel. Although this structure improved the electrical conductivity over prior art solid steel rails, the rail suffers inherent problems associated with the bolted construction including corrosion, electrical hot spikes, and excessive voltage loss. Bolted-on aluminum cladding structures also suffer from high weight, due to the amount of aluminum needed to provide enhanced conductivity, and high power loss due to transfer resistance between the steel rail member and the separate aluminum bar components.
U.S. Pat. No. 3,730,310 to Spiringer also discloses coating the mating surfaces of the steel beam and extrusion with an oxide inhibiting compound. The oxide inhibiting compound reduces oxidation at the mating surfaces and promotes a short term electrically conductive bond between the steel beam and the extrusion. It has been found, however, that the oxide inhibiting compound evaporates and degrades in a relatively short time. Applicant has found such compounds to be ineffective after 2 years of full exposure to the outside environment of heat, cold and humidity which is far less than the twenty five year life expectancy of the rails.
Transfer resistance, also referred to as gap resistance, is directly proportional to the contact or bond between adjoining metals in the rail structure. With bolted-on aluminum rail structures, gap resistance can be significant due to surface imperfections of the steel web, surface irregularities in the extruded aluminum bars, abrasions, nicks or dents in the aluminum caused by handling before and during installation and corrosion or contaminants positioned between the mating metals. High gap resistance between the joining metals liberally encourages electrolytic corrosion in most ambient environments, especially in high humidity environments. As a result of this increased transfer resistance, corrosion and wear, bolted-on aluminum rail structures have a higher replacement cycle than cast or fused rails.
Alternative structures and concepts have been developed by the rail manufacturing industry in the continuing effort to enhance conductivity, minimize power and voltage losses, and ultimately save energy and cost for power conductor rail systems. Conductor rails having an aluminum body with a stainless steel cap to enhance durability were developed in the 1960's. In these rail structures, the aluminum rail body is extruded then capped with steel along the upper flange contact surface to provide extended wear along the contact path where the electrical contact shoe rides along the conductor rail. The cap is secured to the aluminum with mechanical fasteners. Such a capped rail structure is disclosed, for example, in U.S. Pat. No. 3,836,394. Aluminum rails using mechanically bonded stainless steel caps to provide an electrical contact surface are, however, highly disadvantageous because of manufacturing cost and technical deficiencies. Capped aluminum rail structures are nearly four times as expensive per rail foot as a conventional steel rail and nearly twice the cost of composite steel/aluminum rails using prefabricated aluminum extrusion bars bolted or clamped to the steel rail web.
In either the bolted aluminum bar structure or the capped aluminum structure, securing the two metal structures together by mechanical fasteners and the like is undesirable due to the inherent gaps or pockets between the contact surfaces of the joining metals caused by surface imperfections as previously discussed. Furthermore, differential thermal expansion of the metallic components further compromises the metallic contact between the metals and can loosen the mechanical fastening devices employed. Once the fasteners loosen, corrosion is further accelerated by moisture access to and enlargement of the physical junction between the metals. Additionally, extruded aluminum members stress when bent to conform to curved steel rail sections. This stress strains bolted connections.
Processes have been developed to produce steel and aluminum cast composite rails having unified construction to reduce or largely eliminate resistance between the mating steel and aluminum materials and resolve other problems associated with bolted-together composite rails. These so-called "bimetal" rails, and manufacturing processes for making the same, have been developed to combine a ferrous metal, such as steel, with a more conductive metal such as aluminum during the manufacturing process to benefit from the advantages offered from each individual metal and produce a unified construction. U.S. Pat. No. 3,544,737 teaches a bimetal rail and process for making the same wherein aluminum is continuously cast about a steel rail web having preformed apertures to enhance the joining of aluminum and steel and the resultant overall conductivity of the composite rail.
Despite these alternative rail designs, the industry supplying conductor rails still strives to produce a power conductor rail structure which offers minimal electrical resistance while providing the necessary strength and durability to minimize maintenance costs. A typical standard measurement of resistance used in the conductive rail industry is ohms per one thousand feet of connected conductor rail (ohms/1,000 ft.). Typically, unit resistances in conventional conductor rails vary between 0.012 ohms/1,000 feet to 0.002 ohms/1,000 feet. A range of 0.004-0.005 ohms/1,000 ft. is common in existing rail systems using the 150 pounds/yard "New York Rail" employed since the early 1900's in the northeastern United States. The relatively high electrical unit resistance and low efficiency of the "New York Rail" and other conventional rail structures results in a tremendous waste of energy and financial resources. Conventional rail structures commonly provide only a 70% to 75% effective voltage, nearly 30% of the applied voltage is lost due to the high internal resistance of conventional rail structures and other components.
It is, therefore, desirable to have a power rail structure which provides the maximum conductivity and lowest weight per foot of rail while minimizing corrosion, transfer resistance and wear along the surface of engagement with the electrical contact shoe to thereby enhance electrical efficiency and minimize exchange and replacement of rail due to physical maintenance.